- not so Frequently Asked Questions -

A functon which is not defined in a form of y=f(x) but written as
f(x)+g(y)=0 cannot be defined in gnuplot. To draw such an implcit
function, we have to use a parametric representation, or the function is
once defined as a 3-D surface, and find an intersection curve with z=0
plane.

If the function can be defined by using parameters, it is easy to
make a figure. For example, an equation for circle, x*x+y*y=r*r, is
re-defined by x=r*sin(t) and y=r*cos(t). In gnuplot: